Suspended Particle Motion in Turbulence - Effects of Shape and Size on Rotation

The classical theory of particle rotation in Jeffery Orbits has guided how people think about the motion of non-spherical particles. In turbulent flow, additional behaviors arise. We use numerical simulations of particle motion in boundary layer and homogeneous turbulence, as well as laboratory measurements to show how particles rotate in turbulent flow. We vary the shape of particles, covering spheres, rods, discs, and ellipsoids. We also vary the size, from sub-Kolmogorov-scale to Taylor-scale sizes. All particles are suspended due to their neutral buoyancy, i.e. they are not falling through the flow. Results show that the strain-dependence known from Jeffery Orbits disappears in homogeneous turbulence and reappears when mean shear is strong or when particles swim. Following from this is a shape-independence of particle rotation rate; this independence appears for particles of any size, but the mechanism changes when particles cross the Kolmogorov scale.